Quantum modes in DBI inflation: exact solutions and constraints from vacuum selection

TL;DR

This paper derives exact and approximate formulas for the curvature perturbation spectrum in a family of inflation models with variable sound speed, highlighting unique features in DBI inflation related to vacuum selection and potential observable signatures.

Contribution

It provides exact solutions for inflation models with variable sound speed and explores vacuum selection issues in DBI inflation, revealing possible observable effects.

Findings

Exact spectrum expressions for variable sound speed models

Vacuum selection issues in DBI inflation

Potential observable signatures from stringy physics

Abstract

We study a two-parameter family of exactly solvable inflation models with variable sound speed, and derive a corresponding exact expression for the spectrum of curvature perturbations. We generalize this expression to the slow roll case, and derive an approximate expression for the scalar spectral index valid to second order in slow roll. We apply the result to the case of DBI inflation, and show that for certain choices of slow roll parameters, the Bunch-Davies limit (a) does not exist, or (b) is sensitive to stringy physics in the bulk, which in principle can have observable signatures in the primordial power spectrum.